Lapping method and apparatus



Oct. 7, 1952 T. s. LEWIS LAPPING METHOD AND APPARATUS Filed Feb. 8, 1952 INVENTOR: T homers GL6 Patented Oct. 7, 1952 UNITED STATES Parent lorries LAPPING METHOD AND APPARATUS Thomas G. Lewis, Wilmington, DeL, assignor to E. I. du. Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware- Application February 8, 1952, Serial No. 270,544

7 Claims. 1

This invention relates to improvements in method and apparatus for lapping metallic articles, and particularly to a method and apparatus for the lapping of articles fabricated from non-homogeneous or inclusion-bearing metal alloys and metals to develop thereon a high quality surface finish.

Metal alloys and some metals, to varying degrees, are non-homogeneous to abrasion and therefore it is dificult to lap these materials'at a substantially .uniform rate necessary to develop high quality surface finishes with good dimensional control. Typical examples of such materials are the high tungsten, vanadium andchromium content alloy steels, wherein relatively rich carbide concentrations exist at the grain boundaries or at other points. These carbides are usually much harder than the metal of the grain interiors and, in the case of vanadium carbide, at least, may be of a greater hardness than aluminum oxide abrasive itself. An even more serious problem from the standpoint of lapping is that of the impurity inclusions which are found in all commercial grades of metal alloys. These inclusions may comprise small amounts of silicates, sulfides or other substances, or refractory particles picked up from. the smelting furnace, which may be either harder or softer than the primary metal constituent and thus possess a different abrading characteristic, When metal alloys are lapped by the conventional methods now known to the art, inclusions invariably tear away from the surrounding metal, leaving pits behind which reduce the quality of the finish. Usually the dislodged inclusions; join the abrasive and are forced against the work surface one or more times, duringlater passes of the lapping tool, thus producing scratches which additionally mar the finish. ,Yet another cause of pitting is believed to be point welding, which may occur between the tool and the work under the high temperatures. which exist within the small area of contact. The breaking of point welds under the relative movement of the lap and the work tears small pieces of metal away, with resultant pitting, and the free metal particles intermingle with the abrasive and deface the work surface upon being carried into the contact area during subsequent operation. The combined efiect of non-homogeneities, inclusions and point welding has thus hitherto prevented the attainment in metal alloys of lapped surfaces having qualities of the order of 1-2 microinches root-meansquare, which are essential to certain uses of the work-piece of cylindrical shape; v

Fig. 3 is a front elevation of one embodiment.

materials in polymeric material film casting wheels, polishing rolls for coated fabrics and the like. Pure metals, such as electroplated chromium, for example, are generally free of inclusions but are apparently still non-homogeneous to abrasion, since it is very difficult to secure a high quality surface finish thereon while sti retaining good dimensional control. v

I have now found that it is possible to lap metal alloys and also metals to finishesflin the range of l-2 microinches with high dimensional accuracy by forcing a diamond dust abrasive against'the work" with a lignum vitae wood. lap while maintaining a predetermined pressure of contactand aihigh relative speed of translation of. the surfaces'Whereby all areas'are abraded at substantially the same rate and inclusions are not tornfrom the matrix metal. A primary object of this... invention, therefore, comprises providing a method and means for the lapping of articles fabricated from metal alloys or metals to high quality finishes. Other objects include the provision of a method of lapping articles fabricated from metalalloys or metals which is rapid and economical, and which is subject to high dimensional control, so that the finished articles conform to very precise size standards. The manner in which these and other objects of this invention are attainedwill become apparent from the detailed description of this specification and the following drawings, in which? Fig. 1 is a -perspective view of an apparatus adapted to lap a work-piece of cylindrical shape according-to this invention, the outline of the work in set up position being shown by broken line representation;

Fig. 2 is a rear elevational view showing a preferred disposition of lap in relationship to a of lap accordingto this invention; and

Fig. 4 is a sectional view taken along the vertical axis of the lap shown in Fig. 3.

Lapping according to this invention may be conducted either with a free suspended diamondabrasive, or a charged lap may be prepared incorporating the diamondabrasive therein, or a charged lap maybe used in conjunctionwith free suspendedxabrasive, and each of these procedures may have particular advantages, depending on the circumstances, Thus, the use of freesuspendedaabrasives s'o'lely .obviates the. molding of a charged lap as hereinafter described ingreater detail; however, 'apq preciable quantities of diamond are' unavoidably lost during suspension lapping operations, and charged laps are more economical in this respect. The use of free suspended abrasive in conjunction with a charged lap affords the advantage of somewhat greater flexibility in the control of the rate of lapping and may be preferred in instances where this consideration is important.

The embodiment of this invention utilizing free suspended diamond abrasive will be first described, it being understood that much of the detail applicable hereto is equally applicable to the practice with charged laps, either when used alone or together with suspended abrasive.

The lap of this invention may conveniently comprise a single piece of lignum-vitaewood cut transverse the trunk, the ends of the fibers presented constituting a more uniform surface than is obtained when the wood is cut with the grain. Large laps can be built up by assembling a multiplicity of pieces cut across the grain into the lap configuration desired" and holding the-pieces rigidly together byasuitable'device such as that hereinafter described with reference to Figs. 3 and 4, so that a. substantially continuous surface is-provided-for forcingthe abrasive against the work. Since it; isr'necessary to maintain high relative. surface speeds: during lapping, it-is preferred to employlapsofannular shape in the practice of my method, for the reason that this configuration is stable and strong enough to withstandvery high rotational speeds.

:In -Figs.. 3 and- 4, the lignum vitae wood lap 10- comprises a unitarvannulus cut transverse a block 'oi-the wood. so that the ends of -the wood fibersmake upftheworkine. face I l which contaetsithe workduring lapping. The outside circumference of ;lap,l05is: cut: with screw threads l2. engaging .with mating threads: on. the inside periphery of circular metalholder l3, the inner face. of .lap. It being. drawn into tight abutment against the-bottomof theannular recessin the front .face of. holder. 1! 3 and. supported: thereby undertheheavy lapping loading. The rear face of holder 13 isprovidedwith tapped holes 14 for the reception of boltsfor attachment of'the-lap to the flange of: a poweredrotating spindle. The lap -shown in Figs.. 3. and 4 is. adaptedto the lapping of cylindrical work pieces and the working face H is thereforemachined to a plane, smooth surface throughout. '--Where it is desired to lap plane surfaces it ispreferred to machine the outer edge of face I I on a-beveland-maintain holder l3 during operation at an inclination such that the beveled edge is parallel to the work surface, whereupon substantially equal areas of the lap contact the work as in the case-of cylindrical work when the same-loadingpressuresareused.

Lappin according to thi invention can be carried out with. conventional machine shop equipment of the type-shown in-Fig. 1, wherein the cylindrical wo'rk piece 20, in broken line representation, is set upbetween the-centers and turned by a standard grinding machine-2|, such as a-Brown- & Sharpe, Model No.4, Universal Grinding Machine. 'The lap, of the design shown in Figs. 3 and-4, is .powered by aheavy duty motorized spindle 22,.lsuch as .a Pope Machinery Corporation Series 'P-2500 design,.which is substituted forthe'conventionl grinding wheel of the. grinding machinebut ismade to traverse the lap. lengthwise of .thework piece". by operation of grinding machine 21 in the: usual. manner. The'in-feed of thelap; against the. work is conveniently gaugedbymaintaining thehorsepower input to. the snindlecnotor at a suflicientdevel for each of the sizes of abrasive used to maintain high quality lapping at a satisfactory rate. A power demand meter, not shown, of the type represented by the Weston Model 639 Industrial Analyzer is connected in circuit with the motor of spindle 22 to provide the operator with an indication of the existing loading during operation. Itwill be understood that the particular apparatus hereinabove described constitutes but one arrangement of many which can be employed for the purpose, and that numerous other arrangements known to those skilled in the art may be used, depending on the size and shape of the work to be lapped and the practicable speeds at which the work or the lap can be turned.

In the lapping of cylindrical work, good results are obtained when the surface of the lap is disposed substantially tangent to the surface of the work, as indicated in Figs. 1 and 2. The center line of the lap-powering spindle may be coplanar with the longitudinal axis of the work or may be disposed thereabove; however, it is preferred to locate the lap as shown in Fig. 2. wherein the angle between the median line of contact of the lap with the work and the vertical is approximately 45 degs. With this disposition, the right-hand area of contact of the lap passes the work at an inclination.of 45 degs. to the right while the left-hand area of. contact passes the work at an inclination of 45 degs. to the left, thus providinga broken pattern which is desirable in lapping regardless of the particular method involved. Equally good results are obtained when the lap is turned in either a clockwise or counter-clockwise direction as viewed in Fig. 2.

The beneficial results obtained with a lignum vitae wood lap appearto be'due, inpart at least, to the high resin content of-the wood, although other factors, such as the density, modulus of elasticity and other characteristics specific to the material itself, are undoubtedly contributory, and this theory ismerely advanced as the most logical supported bymy observations and is not to be construed as a limitation of the method of this invention'if further investigation adducesa different explanation. In any event, when a fresh lignum vitae lap is put into service it has been observed that little lapping action takes place until sufficient heat is developedat the face of the lap under the combined action of the loading pressure and the relative speed of translation of the surfaces of the lap and the work to cause substantialdiscoloration, or. charring, ofthe face of the lap. Once'charring has occurred, it is-necessary to-maintain the loading and the relative speed of translation within the same general rangeneeessary to-initiate charring in order to obtain the highestylapping rates. therefore, the presence Or absence of charring affords the operator at convenientindex for control of themethod. I

All lapping according to this invention isconducted in the presence of aliquid which serves-as a combined lubricant coolantand purging medium-for theremoval'of metal abraded away during. operation. Vegetable oils, such as olive oil, and animal oils, such as sperm oil, are satisfactory' for all sizes ofdiamond dust' employed, and water has provedsuitable for laps charged with 3200 grit size'diamondi dust: made as hereinafter described. The lapping: of. surfaces from their original state to thewfinalifinishi is a progressive operation, involving theruscof. a series. OfJdiamond dust; sizesrranging; from. relatively coarse at the ployed during each lapping step become. charged.

locally with diamond dust of the particular-size turned at 360.0 R. P. M. and the lap was traversed lengthwise of the work at a speed of 30"=per minute, under which conditions the relative transitory speed between work and lap at the The roll was first reduced to a diameter 0.005- 0.006" above the desired finished diameter, using conventional powered grinding equipment, and

.was then mounted in the apparatus and turned at a speed of 27 R. P. M. The lapspindle wa's 75 lapping With 1600 grit'o'r finer siz'es of diamond used, and it will be understood that a lap which 5 mean diameter-of the lap was calculated to be, has been previously used with a coarser size of about 7600 surface ft./min. Maintaining the diamond dust should not be employed to attain a 1 speeds of the work and lap at the recited values higher quality finish with a finer size unless the through u d d g t l p p iodi al y as charged region is first removed and a fresh sur required by' he development of irregularities in face exposed to the work. This can beaeeomthe working face, the roll was fini hed in four plished conveniently by cutting away-the face of p ate p s WS. the work being cleaned the lap to a depth of about 5%" in a lathe with a thoroughly after pp n W h ch rade o diamond point tool. In addition, lignum Vitae asive: z laps wear unevenly in the course of use, which is S p 1 v evidenced by a periodic bumping sound during Diamond dust of 0 grit Size Sus ended in I operatloni p? more nt 1ceab1e as hlgh olive oil in a concentration of 40 carats/1000 00. spots develop. II) is therefore, preferredto dress was used sumcient suspensionibeing applied the lap to restore an even contacting surface bethe work to maintain a oncentr 1 I ation of 1-2 fore bumping becomes serious enough to cause carats of diamond er Square foot of surf 1 blemishes in the work, and this can be 3.060111;- Thel I t d I k plished by trimming the face of the lap to a depth 1 was mam 'agams e wor under of 0.005"-0.010" with a diamond point tool at inf loadmg of 2 as f i t f the i tervalsof about one hours operation. Upon com mg of i metepmmrcmpwlth Spmdle pletiofi of lapping with a given Size of abrasive, motor, which resulted in almost mmediate charthe surface of the work is cleaned by fiushingor 1mg the lap face The Overall surface gently washing with a solventfsuch as carbon perature 9 the Work s kept .from exqeedmg tetrachloride, followed by a final wash with water about 21.2 by Stoppmg 9 m occasmnauy containing a detergent. .Soft absorbent paper or and Fipraymg c001, clean W cotton are preferably used to dry the lapped sure p was contlnued m thls manner untllvau face and to-remove any remaining traces of abragrmdmg defects 9 m e h h q r sive, after which lapping is started with the next a 0 about 8 m The quality of finish fi si of abrasive v securedwas of the order of 10-15 microinches In the practice of my method, I have found h D was h r ughly Washed with that it is necessary to maintain a relative speed carbon tetrachlorideandthe outer face was cut of translation between. the contacting surfaces away with a diamond point $001 0. a depth-0f of the lap and the work of the order of about 30 about it to p o afresh Surface for S 2 5,000 ft./min., and preferably 7,500-10,000 Y Step 2 1 ft./min. The total loading of the lap against 1 M the work will, of course, depend upon the extent The abraslve employed here was e i dial of the areas in contact andthephysical properm dust ,suspended n Olive Oil in a t ties of the metal alloy or metal from which the non of about 30 carats/1090 h suspension work piece is fabricated. Lapping according to being applied to a n about 1 carat of i this invention has proved particularly effective mOnd Der Sq ft. of work surfa T a n for the high quality finishing. of the relatively s creased to a level of 2 /23 H. P. duri hard stainless steels, of which the following alloys this p the k being Cooled m en y are representative, and the loadings for these ma- 5 described for step Lan after 4 time, t terials, typified by the 440A example hereinafter quality of finish was, found. to be improved to reported in detail, constitute a guide which, sup- 8-10 microinches R. M. S.. Operation was then plemented by observations in the course of lapdiscontinued and the outer face of the lap re-- ping other materials, will enable others to praemoved as hereinabove described to permit. reuse tice my method: 1 in Step 3. I

Analysis, in percent Surface- Material I E 'E iX 0 M11 P S Si Cr Ni Cu Fe ness Armco Iron 31104.. 0.06 0. 0. 013 0.012 0.30 16.34 4. 00 3.90 Balance 30 1 AISI 440A 0.60 1.0 1.0 15.0 1.25 Balance 55 C Mn PV .8 Si Gr Mo V Fe LatrobeElectricSteel Oo/sBR- 2.4 0.40 0. 40 12.8 1.1 4.0 Balance A 6" dia. solid roll 12" long'fabricated from Step 3 commercial quality 440A steel of theanalysis and hardness tabulated was lapped according to 6 q dust'of 1600 S1Ze- Suspended m this invention in the following manner, using the o we 011 m a m about 20 apparatus shown in Figs. 1-4. The lap comcarats/1009' was usedfia quantlty of about 1. prised a 1" thick unitary lignum vitae wood diamond/Sq of Work Surface being ring of 7" inside diameter and 9" outside diame mamtamed- T 'lozjldmg was incmased to a ter, thus providing working face of 1" width. level o '3 o slig tly greater, and cooling was effected by occasionally directing a small stream of cool, clean water on the work without discontinuing operation. It was found that cooling by this procedure had no adverse efiects when and hadthe advantage that itshortenedthe time required somewhat. After 4 hours of operation the quality of finish attained was. about 3-7 mlcroinches R. M. S. The lap was then'removed andprovided -with a fresh surfaceas describ'ed for steps 1- and 2.

Step 4 I In this final operation, diamond dust of 3200 gritsize was used in aiconcentration of -15 carats/1000 00., between; 05-0775 carat ofdiamond being maintained/sq. ft. of worksurface. The loading of the lap against the work was increased to the level of about 4 H. P. and the work was cooled as described in Step 3. After 4 hours operation, a surface finish of a quality of 1-2 microinches R.- M. S. was obtained and the work was found to be within the dimensional toler-' ances specified. Inclusions and non-homogeneities in the work piece were apparently evenly abraded away together with the surrounding metal, and there were no scratches-such as would be caused from inclusions or weld metal tearing away and joining the abrasive.

' A different cylindrical roll of the same dimensions as that hereinabove described for the lapping of 440 A alloy was electroplated with chromium to a thickness of about 0.012", after which it w as ground to a preliminary finish, thus reducing the thickness of the chromium to about 0.010. The hardness of the chromium plate was measured and found to be 60 Rockwell C. This roll was then lapped by the identical procedure hereinabove describedforthe 440 A alloy steelpe'xcept that the quality of finish attained upon the completion of the lapping of the chromium-plated roll with-i600 grit size diamond dust was found to be of the order of l-2 microinches R. M. S., and operation was therefore terminated without lapping with 3200 grit size abrasive.

-1frbnr the foregoing it will be understood that, after protracted use, natural wood lignum vitae laps become suil'icientlyeharged with diamond so that a certain amount of lapping can be 'conducted without the use of additional suspended diamond, or at leastby theme of somewhat more dilute concentrations. However', it is preferred to make up charged laps by molding lignur'n vitae wood flour under heat and pressure, supplying the diamond by the incorporation of pre determined amounts of diamond dust uniformly distributed therein. The molded lignum vitae is completely homogeneous in structure and uncharged molded material may,'. in fact, be used to advantage as a substitute for the'natural wood in suspension lapping where an exceedingly uni form lapping contact is desired. Furthermore' natural lignum vitae frequently develops 'cracksf when exposed to dry air which render the piece unsuitable for use in lapping, whereas the molded material is considerably more stable under changing humidity conditions.

The lignum vitae wood flour can be. prepared particle size of. 0.001" orsomewhat finer. The lignum vitaeflour is molded byhsubjecting the material to a': pressure ofratleast 5000 lbs/sq; in. and maintaining the m01d 2:1'1d".itS contents at a temperature of 375-400 F. -whi-le: undertthis pressure for a period of one hour. At this pressure. and temperature the lignum vitae-resin;

..of 3200 grit .size.

flows freely around the wood particles and, on cooling, a molded block is obtained having a specific gravity of 1.38 to 1.42, within the same range asthat'of the natural wood. The duration of the heating and the temperature employed are both rather critical, insufficientheating' re-' sulting'in a weak bond, while excessive heating produces brittle compacts. Molds provided with exteriorly disposed channels for the circulation of heating oil have proved entirely satisfactory, the female component of a mold for the production of annular blocks constituting a metal piece machined with a circular groove of the dimensions desired in the molding; while the male component-consisted 'of a metal ring sized to fit slid ablywithin the formative groove. If desired, the lig'num vitae can be molded integral with the ring part of the mold, avery strongbond being obtained if the ring member is provided with suitable recesses anchoring a portion of the wood filling to themetal. In one construction the entire face of themolding ringwas machined at a pitch of 0.10" with a succession of concentric grooves 0.05" deep and 0.05"

' wide and the top surface of the metal was pressed inwardly with a knurling tool applied radially with respect to the ring. This treatment produced groove cross sections of somewhat smaller width at the top than at the bottom, providing a strong key structure for attachment of the wood. integrally bonded woodmetal laps are of sufficiently high strength so that no additional supports for the wood are necessary and the 'lap may be secured to the turning spindle bymerely bolting the metal backing to .thespindle flange directly.

Molded laps are conveniently formed by charging the female part of the mold with successive increments of the wood flour and pressing after each addition until the desired thickness is obtained, then carrying out the heating while retaining under pressure; Charged laps are made by incorporating diamond dust of the appropriate grit siz'e uniformly distributed throughout the flour; In a specific case, an annular lap integrally bonded to a metal backing plate was prepared havinga 7" inside diameter, a- 9" outside diameter and a thickness of part of the diewas loaded in six separate increment's, the diamond charge being limited to the first wood fiour increment, which weighed 0.63

ounce, and comprising 5 carats of diamond dust This lap performed well and there-was no indication that the strength of which suspended diamond is utilized. In fact, it appears that charged laps have an enhanced .abrading. action over the natural wood'used-in -conjunction-with suspended diamond, in that laps charged with finer sizes of diamond, such as that the'surface tension of water is appreciably lower than'that of most oils.

. From the foregoing it will .be understoodthat my invention may be modified in numerous ways obvious to those skilled in the art without departing from theessential spirit; wherefor it The female 9 is intended to be limited only by the scope of the following claims.

What is claimed is:

1. A method for the lapping of an article fabricated from a metal alloy or metal comprising forcing a diamond dust abrasive against said article in the presence of a liquid with a lignum vitae wood lap while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure sumcient to char the surface of said lap in contact with said article.

2. A method for the lapping of an article fabricated from a metal alloy or metal in two separate contactlngs of the lap with said article wherein the direction of translation of the lap with respect to the surface of said article during one of said contactings is maintained substantially normal to the direction of translation of the lap with respect to the surface of said article during the other of said contactings, each of said contactings comprising forcing a diamond dust abrasive against said article in the presence of a liquid with a lignum vitae wood lap while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure suflicient to char the surface of said lap in contact with said article.

3. A method for the lapping of an article fabricated from a metal alloy or metal comprising forcing a diamond dust abrasive suspended in a liquid against said article with a lignum vitae wood lap while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a load- I ricated from a metal alloy or metal comprising contacting said article with a diamond-charged lignum vitae wood lap in the presence of a liquid while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure suflicient to char the surface of said lap in contact with said article.

6. A method for the lapping of an article fabricated from a metal alloy or metal in a succession of steps employing progressively finer graded particle sizes of diamond dust abrasive comprising forcing each of said sizes in turn against said article in the presence of a liquid with a fresh lignum vitae wood lap while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure sufficient to char the surface ofsaid lap in contact with said article, and cleaning the surface of said article after each said step before beginning the next succeeding step.

7. A charged lap comprising molded lignum vitae wood flour incorporating a diamond dust abrasive substantially uniformly distributed throughout the region wherein contact is made with the work. i

THOMAS G. LEWIS.

No references cited. 

